The below is a link of a series of you tube lectures given by Dr Tadashi Tokieda on Topology & Geometry for the people who think topology/Geometry is a difficult subject. I bet you will really change your mind after watching these lectures.
Dear readers, I am posting a link found via Google about the Closure of a Linear Operator. The author characterises the existence of the closure of a linear operator. The proof of the characterization is clear, if some one is unclear about the proof, he/she can discuss me.